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2L - Immunotoxicity - genotoxicity - ED (WEP/97) Gonadal sex differentiation in the fathead minnow, sheepshead minnow and zebra fish. Wallis, Nicola1, Smith, Fiona1, Panter, Grace2, Hutchinson, Tom2, 1 Syngenta Central Toxicology Laboratory, Alderley Park, Cheshire, UK2 AstraZeneca Global Safety, Health & Environment, Brixham Environmental Laboratory, Devon, UK ABSTRACT- Exposure of fish to endocrine disrupting chemicals (EDCs) during the period of gonadal development leads to skewing of the population sex distribution, intersex fish (possessing ovo-testes) and abnormalities in the gonadal ducts. Using standard OECD laboratory conditions (temperature 25+/-1C), for fathead minnows Pimephales promelas, sheepshead minnows Cyprinodon variegatus) and zebrafish Danio rerio, a baseline (clean water) study was performed to assess the maturity of the gonads at 30, 60 and 90 days post-hatch (dph), in order to determine the optimal time point for gonadal histology evaluation in ecotoxicity studies. For each species, 20 fish per time-point were sampled and sectioned using methods developed by our group to allow comprehensive evaluation of all gonadal zones and associated duct structures. Our results show that in all three species, the female gonad developed earlier than the male, with ovarian follicles identifiable in a proportion of fish by 30 dph, and all females by 60 dph. Male gonads were identifiable in a small proportion of fish by 60 dph and in all male fish by 90 dph (in all three species). Although gonadal sex was easily determined by 90 dph, reproductive maturity (presence of mature spermatid populations), was not detectable in fathead minnows at 90 days. Sheepshead minnows have a single central gonad, unlike the other species that have paired gonads. In conclusion, by 90 dph it is possible to definitively determine the gonadal sex of all three species allowing detection of EDCs affecting gonadal sex differentiation. This evaluation would not be possible at earlier time-points, as a proportion of the fish have indeterminate gonads. The technical processing also allows for evaluation of the EDC-sensitive accessory ducts. Key words: fathead minnow, gonad histology, sheepshead minnow, zebrafish |
